II. Division Coniferophyta (Conifers)

A. General information

1. Conifers are the largest group of gymnosperms

2. The more than 575 species of conifers include pines (Pinus), firs (Abies), spruces

(Picea), hemlocks (Tsuga), cypresses (Cupressus), Douglas firs (Pseudot-suga), and junipers (Juniperus)

B. Pines

1. Most pines are found in the northern hemisphere, although they have been exten-

sively planted in the southern hemisphere

2. Needles are a distinctive characteristic of the pines

a. They usually appear in clusters known as fascicles

b. They have adapted to winter conditions, when the ground water is frozen

and the needles are exposed to drying winds

1. The needles have a thick epidermis with a heavy cuticle

2. Several layers of thickened cells just below the epidermis (called the

hypodermis) further reduce the potential for water loss

3. Stomata are located in pits to shield them from the wind's drying action

4. Veins are surrounded by endodermis, which is another layer of large

cells that prevents excessive water loss

(5) Mesophyll cells are packed tightly together; they do not have the obvi-

ous air spaces seen in the spongy mesophyll cells of deciduous leaves

(6) The presence of resin canals, which are produced in response to in-

jury, may prevent insect and fungal damage

c. The fascicles drop off two to five years after maturation; they are lost a few at

a time rather than all at once as with the leaves of deciduous trees

3. The xylem tissue of conifers contains tracheids as the conducting cells instead of

the vessels found in angiosperms

a. Because the secondary xylem of conifers lacks thick-walled vessels and fi-

bers, conifer woods are labeled soft woods

b. The resin canals in the secondary xylem serve to inhibit fungal infections

and prevent damage by plant-eating animals (herbivores)

4. The thick bark of conifers contains large amounts of secondary phloem; the

phloem contains albuminous cells that perform the same function as the com­panion cells of angiosperms

5. Pines almost always have mycorrhizae attached to their roots

6. As a heterosporous plant, the pine begins its reproductive life cycle with the pro-

duction of two types of spores—male microspores and female megaspores

(see Life Cycle of the Pine)

a. Microspores are produced in a process called microsporogenesis

(1) Male cones, which are typically 1 to 4 cm long, usually are found in

clusters of 50 or more on tips of the lower branches

(2) Male cones last for only a few weeks, during which time each diploid

microspore parent cell, or microsporocyte, undergoes meiosis to form four haploid microspores

(3) Each haploid microspore undergoes mitosis and matures into a pollen

grain, which consists of four to five cells and a pair of external air sacs or wings

(4) The pollen grain contains a tube cell (which forms the pollen tube); a

generative cell (which, after dividing, produces sperm nuclei); and two

to three prothallial cells (which are the remnant vegetative cells of the gametophyte generation)

(5) Vast numbers of pollen grains are shed from the cones and dispersed

by the wind to pollinate the female cones

(6) To aid wind pollination, pollen grains have two wings that develop from

the microspore cell wall

(7) Once the pollen grains are released, the male cones dry up and drop

from the tree b. Megaspores are produced in a process called megasporogenesis

(1) Female cones are larger than male cones and develop in the spring

1. These woody cones persist for long periods of time

2. They are generally located on the upper branches

(2) The female cone consists of a number of modified leaflike organs

called sporophylls that bear sporangia and serve as shelves on which the seeds develop

(3) Two ovules are located on each sporophyll

(a) Each ovule has an opening, called a micropyle, through which

the pollen tube enters, and a megasporangium (also called the nucellus) surrounded by two integuments (outer tissue layers)

(b) Each megasporangium contains a megasporocyte, or mega-

spore parent cell

(4) The megasporocyte produced within the ovule undergoes meiosis to

form four haploid megaspores; three disintegrate and one develops into the female gametophyte

(5) The female gametophyte grows slowly, taking approximately 13 months

to reach maturity

(6) Once mature, the female gametophyte consists of several thousand

cells and two to six archegonia, each containing a single egg

(7) Typically all eggs in the archegonia are fertilized and begin to develop

into embryos (a phenomenon called polyembryony, or multiple em­bryo production); only one embryo develops fully and the others spon­taneously abort 7. Fertilization and seed development occur within the female pine cone

a. In the first spring of the reproductive cycle, the immature cone scales spread

apart; pollen grains (carried by air currents) sift down between the sporo­phylls and are caught in sticky secretions near the micropyle

b. The pollen grain germinates within the micropyle to produce a pollen tube

c. As the pollen tube grows toward the archegonium, the generative cell of the

pollen grain divides, producing a sterile cell and a spermatogenous cell

(1) The spermatogenous cell further divides to form two sperm; the sterile

cell, whose function is unknown, disintegrates

(2) The mature male gametophyte consists of the pollen grain and the two

sperm

d. About 15 months after pollination, the pollen tube reaches the archegonium

and releases its contents

e. One sperm unites with the egg and the other disintegrates

f. The zygote formed from the union develops into an embryo

g. The developing embryo is surrounded by integument, which consists of rem-

nant sporophyte tissue and forms a seed coat

h. The mature embryo consists of an epicotyl, hypocotyl, radicle, and multiple cotyledons

8. In pines, the seed matures approximately 12 months after fertilization, and the

complete life cycle takes about two years

a. Male and female cones are formed during the summer of the first year

b. In the spring of the following year, microspores and megaspores are pro-

duced by meiosis

c. Pollination occurs during the early summer, and fertilization takes place in

the late spring of the second year

d. By the summer of the second year, the embryo develops

e. Seeds are produced and dispersed in the fall of the second year

9. Seeds can remain dormant for many years, and some may be embedded in the

mature cones for six years or more

10. Pine seeds are shed from the cones during the autumn of the second year

a. When mature, the cone scales of most pine species separate, and the

winged seeds are carried through the air by wind currents

b. In species whose seeds do not have wings, the seeds may be dropped by

birds during flight or when attempting to eat

c. In some species, such as the lodgepole, jack, and knobcone pines, the

cones open only when exposed to heat (a fire, for example), which melts the resin that holds the cone scale closed

11. Other conifers lack the needle clusters of pines and may have slightly different re-

productive cycles

a. The yews (family Taxaceae) do not produce woody female cones; instead,

the ovule is at least partially surrounded by a fleshy, cuplike covering

b. The Norfolk Island pine (Araucaria excelsa) lacks the needle clusters of other

pines and bears its needles singly along stems and branches; it is native to the southern hemisphere and is a common houseplant